This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-140211, filed May 20, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a torsional rocker used in an optical scanner, angular acceleration sensor, etc.
A torsional rocker, which is manufactured by a semiconductor manufacturing process, for example, comprises a movable element which is supported by means of a torsion spring structure or elastic member. An optical scanner that utilizes a torsional rocker of this type is described in Jpn. Pat. Appln. KOKAI Publication No. 10-123449. FIG. 32 shows an outline of this optical scanner.
The optical scanner comprises a movable element 12, a pair of torsion spring structures or elastic members 16 and 18 for rockably supporting the movable element 12, and a stationary element 14 for fixing the structures 16 and 18. Each of the torsion spring structures 16 and 18 is composed of one leaf spring, and the movable element 12 includes a coil 20. These elements are formed integrally with one another by utilizing the semiconductor manufacturing process. On either side of the movable element 12, two permanent magnets 22 and 24 are symmetrically fixed to the stationary element 14 by adhesive bonding.
As current is applied to the coil 20, it produces Lorentz force in a magnetic field which is formed by means of the permanent magnets, whereupon the movable element 12 rocks around the leaf springs 16 and 18. A mirror is formed on the back surface of the movable element 12 (opposite from the surface on which the coil 20 is formed), and a light beam that is reflected by the mirror is scanned as the movable element 12 rocks.
In order to improve the scanning speed of the optical scanner of this construction, it is necessary to increase the stiffness of the leaf springs which support the movable element 12. If the width, thickness, and length of each leaf spring are w, t, and 1, respectively, the torsional stiffness of the spring is proportional to wt3/l. Accordingly, the stiffness of the elastic members can be improved by increasing the width or thickness of each spring or reducing the length.
Although the stiffness of the elastic members can be improved most effectively by increasing the thickness of each spring, a thick film cannot be formed with ease in the semiconductor manufacturing process. If a thick film can be formed, moreover, a substantial residual stress is often generated in the film, possibly exerting a bad influence upon the flatness of the mirror which is formed on the rockably movable element 12.
If the length is reduced, on the other hand, stress generated in the leaf springs increases as the springs undergo torsional deformation that is caused by the same torque. If the stress exceeds a given value, the springs may be broken. Accordingly, the leaf springs cannot be made very short. Actually, therefore, the stiffness of the leaf springs can be increased only by increasing the width.
If the width of the leaf springs is increased, however, tensile stress is generated in portions that are distant from the swing axis of the torsional deformation. Accordingly, the external torque-torsional angle characteristic of the leaf springs inevitably becomes non-linear. Since the stiffness increases as the torsional angle widens, therefore, a desired torsional angle cannot be obtained. If the non-linear leaf springs are resonated, moreover, the resulting frequency components include components of frequencies that are equal to integer multiples of the resonance frequency, so that unnecessary high-order resonance may possibly be caused.
The object of the present invention is to provide a torsional rocker, in which the stiffness of torsion spring structures is improved and the non-linearity of the torque-torsional angle characteristic of the spring structures is restrained.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.